President Vladimir Putin vowed Saturday to defend Russia against an armed rebellion by mercenary chief Yevgeny Prigozhin, who led his troops out of Ukraine and into a key city south of Moscow.

The uprising, which Putin called “a stab in the back,” was the biggest threat to his leadership in over two decades in power.

The private army led by Prigozhin appears to control the military headquarters in Rostov-on-Don, a city 660 miles (over 1,000 kilometres) south of Moscow that runs Russian offensive operations in Ukraine, Britain's Ministry of Defence said in an intelligence briefing.

As the fast-moving events unfolded in Russia, Ukrainian President Volodymyr Zelenskyy said Moscow is suffering “full-scale weakness” and that Kyiv was protecting Europe from “the spread of Russian evil and chaos.”

In his address, Putin called the uprising by Prigozhin, whom he did not mention by name, a “betrayal” and “treason.”

“All those who prepared the rebellion will suffer inevitable punishment,” Putin said. “The armed forces and other government agencies have received the necessary orders.”

Prigozhin called himself a patriot.

“Regarding the betrayal of the motherland, the president was deeply mistaken. We are patriots of our homeland," he said in an audio message on his Telegram channel.

He said his fighters would not turn themselves in at the request of Putin, as “we do not want the country to live on in corruption, deceit and bureaucracy.”

Prigozhin's private military contractor, known as Wagner, has been fighting alongside Russian troops in Ukraine. It wasn't immediately clear what his aims were, but the rebellion marks an escalation in Prigozhin's struggle with Russian military leaders, who he has accused of botching the war in Ukraine and hamstringing his forces in the field.

“This is not a military coup, but a march of justice,” Prigozhin said.

Prigozhin confirmed Saturday he and his troops reached Rostov-on-Don after crossing the border from Ukraine.

He posted a video of himself at the Russian military headquarters in Rostov and claimed that his forces had taken control of the air field and other military facilities in the city. Other videos posted on social media showed military vehicles, including tanks, on the streets.

Prigozhin said his forces faced no resistance from young conscripts as they crossed into Russia, saying his troops “aren't fighting against children.”

“But we will destroy anyone who stands in our way,” he said in one of a series of angry video and audio recordings posted on social media beginning late Friday. “We are moving forward and will go until the end.”

Putin condemned the rebellion, which comes at a time when Russia is “fighting the toughest battle for its future” as western governments heap sanctions on Moscow and arm Ukraine.

“The entire military, economic and information machine of the West is waged against us,” Putin said.

Russia's security services, including the Federal Security Service, or FSB, called for Prigozhin's arrest after he declared an armed rebellion late Friday.

In a sign of how seriously the Kremlin took the threat, authorities declared a “counterterrorist regime” in Moscow and its surroundings, allowing restricted freedoms and enhancing security in the capital.

It was not immediately clear how Prigozhin was able to enter the southern Russian city or how many troops he had with him.

Prigozhin said his aim was to punish Defence Minister Sergei Shoigu after Russian government forces attacked Wagner field camps in Ukraine with rockets, helicopter gunships and artillery.

Gen. Valery Gerasimov, chief of the General Staff, ordered the attacks following a meeting with Shoigu at which they decided to destroy Wagner, Prigozin said.

He said Wagner's forces shot down a Russian military helicopter that fired on a civilian convoy, but there was no independent confirmation.

Prigozhin said he had 25,000 troops under his command and urged the army not to offer resistance.

After Putin's address, in which he didn't mention concrete steps to suppress the rebellion but rather called for unity, officials and state media personalities in the country sought to publicly reiterate their allegiance to the Kremlin and urged Prigozhin to back down.

Vyacheslav Volodin, speaker of Russia's lower house of parliament, said lawmakers “stand for the consolidation of forces" and support Putin, adding that “Wagner fighters must make the only right choice: to be with their people, on the side of the law, to protect the security and future of the Motherland, to follow the orders of the commander-in-chief.”

Foreign Ministry spokeswoman Maria Zakharova echoed Volodin's sentiment, saying in a Telegram post that “we have one commander in chief. Not two, not three. One."

Ramzan Kadyrov, the strongman leader of the Chechnya region who used to side with Prigozhin in his criticism of the military, also expressed his full support of Putin's “every word.”

“We have the commander in chief, elected by the people, who knows the situation to the slightest detail better than any strategist and businessman,” Kadyrov said. “The mutiny needs to be suppressed.”

While the outcome of the confrontation was still unclear, it appeared likely to further hinder Moscow's war effort as Kyiv's forces probed Russian defenses in the initial stages of a counteroffensive. The dispute, especially if Prigozhin were to prevail, also could have repercussions for Putin and his ability to maintain unity.

The Wagner forces have played a crucial role in Ukraine, capturing the eastern city of Bakhmut, where the bloodiest and longest battles have taken place. But Prigozhin has increasingly criticised the military brass, accusing it of incompetence and of starving his troops of munitions.

In his Telegram channel, Zelenskyy noted the rebellion and said “anyone who chooses the path of evil destroys himself.”

“For a long time, Russia used propaganda to mask its weakness and the stupidity of its government. And now there is so much chaos that no lie can hide it,” he said. “Russia's weakness is obvious. Full-scale weakness. And the longer Russia keeps its troops and mercenaries on our land, the more chaos, pain and problems it will have for itself later.”

Prigozhin's actions could have significant implications for the war. Orysia Lutsevych, the head of the Ukraine Forum at the Chatham House think tank in London, said infighting between the Defence Ministry and Wagner will create confusion and potential division among the Russian forces.

“Russian troops in Ukraine may well now be operating in a vacuum, without clear military instructions, and doubts about whom to obey and follow," Lutsevych said. “This creates a unique and unprecedented military opportunity for the Ukrainian army.”

Heavy military trucks and armoured vehicles were seen in several parts of central Moscow early Saturday, and soldiers toting assault rifles were deployed outside the main building of the Defence Ministry. The area around the presidential administration near Red Square was blocked, snarling traffic.

But even with the heightened military presence, downtown bars and restaurants were filled with customers. At one club near the headquarters of the FSB, people were dancing in the street near the entrance.

Prigozhin, whose feud with the Defence Ministry dates back years, had refused to comply with a requirement that his forces sign contracts with the ministry before July 1. He said Friday he was ready for a compromise but “they have treacherously cheated us.”

“Today they carried out a rocket strike on our rear camps, and a huge number of our comrades got killed,” he said. The Defence Ministry denied attacking the Wagner camps.

“The evil embodied by the country's military leadership must be stopped,” he shouted.

Col. Gen. Sergei Surovikin, the deputy commander of the Russian forcesin Ukraine, urged the Wagner troops to stop any move against the army, saying it would play into the hands of Russia's enemies who are “waiting to see the exacerbation of our domestic political situation.”

In Washington, the Institute for the Study of War said “the violent overthrow of Putin loyalists like Shoigu and Gerasimov would cause irreparable damage to the stability of Putin's perceived hold on power.”

At the White House, National Security Council spokesperson Adam Hodge said the administration was monitoring the situation and will be consulting with allies and partners on the developments.